Proliferative diseases, for example cancer, are characterized by the uncontrolled growth of abnormal cells. Current treatments of proliferative diseases include surgery, radiation, chemotherapy, hormone-based therapy and/or immunotherapy. A number of these treatments, particularly chemotherapy, utilize anti-proliferative drugs that limit the spread of the abnormal cells. However, these drugs are typically indiscriminate in their ability to kill cells, affecting both normal and abnormal cells. To address this problem, various approaches to targeted drug delivery have been explored, including the use of conjugates of tumor-targeted probes (such as antibodies or growth factors) with toxins, to selectively target abnormal cells. Antibody drug conjugates (ADCs) are compounds composed of an antibody that is linked, via a chemical linker, to a cytotoxic agent. Such compounds leverage the antibody's binding specificity for its target to deliver a cytotoxic agent to an abnormal cell. Thus, there is a need for anti-proliferative compounds and their conjugates.